This project is designed to test the hypothesis that adenosine, a potent vasodilator which appears in ischemic muscle, plays a role in regulation of coronary flow, and whether release of adenosine, inosine and hypoxanthine is a useful indicator of myocardal ischemia. The studies are facilitated by a spectrophotometric method of measurement accurate to 0/1 nanomole of adenosin. In animals, the sequential relationship of adenosine release to lactate production and to reactive hyperemia after brief ischemia will be examined to determine whether a temporal relationship exists; in addition, the levels of nucleosides in regional and coronary sinus blood will be measured after longer ischemia to determine whether it is a versible phenomenon. In man, the levels of appearance of nucleosides in CS blood before, during and after pacing-induced angina will be determined during routine clinical cardiac catheterizations. In addition, levels of nucleosides and lactate will be measured in CS blood collected intraoperatively during ischemic periods, and by cannulation of the C.S. in selected patients with unstable angina or acute myoocardial infarction, during therapeutically indicated studies.